This invention relates to a current mirror circuit, and more particularly to a current mirror circuit which has high accuracy in its current mirror transfer ratio even when constructed of P-N-P transistors.
To obtain an output current matched to the input current, a current mirror circuit is often used. The improved Wilson current source which is a conventional circuit for use as a current mirror circuit is well-known.
FIG. 1 shows the equivalent circuit of the improved Wilson current source. As shown in FIG. 1, the current source comprises four matched monolithic bipolar transistors Q1, Q2, Q3 and Q4. The base electrodes of the matched transistors Q1 and Q2 are connected in common, and the collector electrode of the transistor Q1 is connected to the commonly connected base electrodes, and also connected to the input current source 1.
The collector electrode of transistor Q2 is connected to the load 2, and provides the output current Io. The emitter electrodes of transistor Q1 and Q2 are connected to the collector electrodes of matched transistors Q3 and Q4, respectively. The base electrodes of transistors Q3 and Q4 are connected in common, and the common base electrode is connected to the collector electrode of transistor Q4. The emitter electrodes of transistors Q3 and Q4 are connected to a first power supply terminal 3, and the input current source and the load 2 are connected to a second power supply terminal 4.
This current source has the advantage that, when the four transistors are matched, the collector-emitter voltages V.sub.CE of transistors Q3 and Q4 are equivalent to their base-emitter voltages V.sub.BE. Therefore the relationship between the collector current of transistors Q3 and Q4 is free from the Early effect (the phenomenon that the collector current varies approximately linearly with collector-emitter voltage for fixed base current).
So the current mirror transfer ratio of this source is very good. The current mirror transfer ratio .lambda. of this source is given by ##EQU1## wherein Ii: input current, namely the collector current of transistor Q1,
Io: output current, namely the collector current of transistor Q2, PA0 .beta.: common emitter current gain.
In this equation, the second term of the right side shows the error, and the value of the error is inversely proportional to the square of the .beta.. The .beta. of an NPN transistor is high, but the .beta. of a PNP transistor is much smaller than the .beta. of NPN transistors, usually a tenth of the .beta. of NPN transistors. Therefore, when this current source is constructed of PNP transistors, the error value is large, and a well-matched output current to input current is not expected.
FIG. 2 shows another conventional current mirror circuit which is constructed using PNP transistors. In this circuit, the base electrodes of matched PNP transistors Q5 and Q6 are connected in common, and their respective emitter electrodes are connected in common to terminal 7, and the collector electrode of transistor Q6 is connected to a first power supply terminal 9 through load 8, and carries the output current Io. The emitter electrode of the PNP transistor Q7 is connected to the common base electrodes of transistors Q5 and Q6, the base electrode of transistor Q7 is connected to the collector electrode of transistor Q5, and the collector electrode is connected to a second power supply terminal 6. In this circuit, when the base current of transistor Q7 is negligible (but as mentioned above the .beta. of the PNP transistor is not large, and the base current is not usually negligible) the output current Io is equal to the input current Ii.
But in this circuit, the collector-emitter voltage of the transistor Q6 is not fixed. Namely, when the voltage of the output terminal 9 is varied, the collector-emitter voltage V.sub.CE of transistor Q6 is also varied. So, by the Early effect (above-mentioned), the collector current of the transistor Q6 is varied. Therefore the relationship between the input current Ii and the output current Io is variable. So high accuracy is not expected.
Accordingly, it is an object of this invention to provide a current mirror circuit which has a good current mirror transfer ratio even if constructed using PNP transistors. Another object of this invention is to provide a current mirror circuit which is free from the Early effect.
These and other objects of this invention will become apparent to one skilled in the art with a reading and understanding of the following description of the invention and claims.